Pheromonal chirality and integrity of aggregation response in southern species of the bark beetle Ips sp.

Abstract

PHEROMONAL communication by which southern pine engraver beetles signal aggregation and colonisation involves three terpene alcohols, ipsdienol (2-methyl-6-methylene-2,7-octadien-4-ol), ipsenol (2-methyl-6-methylene-7-octen-4-ol) and cis-verbenol first isolated from Ips paraconfusus Lanier in California¹. The absolute configurations of these compounds are (1S, 4S, 5S)-cis-verbenol², (S)-(−)-ipsenol³, and (S)-(+)-ipsdienol⁴. I. grandicollis (Eichhoff) responds to (S)-(−)-ipsenol⁵ and seems to produce it as its only pheromonal compound. I. calligraphus (Germar) uses (S)-cis-verbenol and ipsdienol as an aggregation pheromone, and I. avulsus (Eichhoff) aggregates in response to ipsdienol and (S)-(−)-ipsenol, apparently the combination of a pheromone and a kairomone⁶. Although frass produced by I. paraconfusus attracted I. grandicollis in field tests, it failed to aggregate either I. calligraphus or I. avulsus. Similarly, I. calligraphus producing (S)-cis-verbenol and ipsdienol did not aggregate I. paraconfusus when frass of I. grandicollis was added as a source of (S)-(−)-ipsenol. Laboratory bioassay has confirmed the lack of cross attraction between I. paraconfusus and I. calligraphus⁷. Furthermore, when synthetic pheromones were used in field bioassay, I. avulsus, I. grandicollis and I. calligraphus readily responded to racemic formulations of their own pheromones, but there has been only one report of field attraction of I. paraconfusus with racemic pheromonal components⁸. We now offer an explanation for these phenomena which involves the field responses of southern Ips sp. to the enantiomers of ipsdienol⁹.